Magnetic sight gage sensor

ABSTRACT

A magnetic sight gage sensor comprises a sensor for determining the liquid level in a process vessel utilizing a magnetic float on the interior of a tube. The tube is connected at different levels to a process vessel and contains a liquid that is maintained at the same level as the liquid inside the process vessel. A magnetostrictive displacement transducer is coupled to the tube or sight glass on the exterior of such glass, so it is out of the process fluid. The transducer provides signals indicating the position of the float relative to a reference position on the transducer. The tube or sight glass is made of a non-magnetic material in order to allow transmission of the magnetic field from the float to the transducer.

BACKGROUND OF THE INVENTION

The present invention relates to the use of a precision displacementtransducer for determining levels of liquid in a process vessel bymounting a magnetostrictive transducer onto the exterior of a sightglass having a liquid in communication with the liquid inside theprocess vessel, and a float that floats at the level of the liquid. Thefloat contains a magnet that will provide a magnetic field that servesas the input signal for determining distances of the displacementtransducer.

Various sight gage liquid level indicators have been advanced, includingliquid level indicators that carry a magnet on the inside of a sightglass or tube, and which in turn cause a magnetically coupled indicatorto move in a separate glass tube outside of the sight glass to establisha visual indication of liquid level in the process vessel. In someinstances, the magnetic float has been utilized for continuous remotelevel transmission using electric signals that provide an output as afunction of the float movement.

However, these products are imprecise, and do not provide an accuratecontinuous analog output of liquid levels for constant monitoring, andfor inputs into various process controls.

While the magnetostrictive transducers have been used with floats thatsurround the sensing portions of the transducers, the present inventionkeeps all of the transducer components away from the process liquidwhile providing the benefits of precise and continuous indications ofliquid levels.

SUMMARY OF THE INVENTION

The present invention relates to a magnetic sight gage sensor utilizinga magnetostrictive displacement transducer positioned adjacent a bypasstube or column partially filled with liquid that rises and falls inaccordance with the liquid level in a separate large process vessel, andin which a float carrying a magnet is placed. The magnetostrictivesensor will be affected by the magnetic field passing through a magnetictransmissive material wall housing the float, and in accordance withknown principles will provide a precise displacement signal from areference point to determine, on a continuous basis, the level of liquidin the process vessel. The installation is simple, low cost, and thesensitive parts of the assembly are maintained isolated from corrosive,high temperature and high pressure process fluids. The invention can beapplied to existing sight gages, which have magnetic floats on theinterior, to greatly enhance the accuracy of liquid level determination,and the response time of the sensor for very precise control ofprocesses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a process vessel having amagnetic sight gage sensor known in the prior art installed thereon;

FIG. 2 is an enlarged schematic sectional view of a float and lineardisplacement transducer used in the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a process vessel or tank indicated schematically at10 is used for containing a process fluid used in an industrial process.The liquid indicated at 11 inside the tank has a level 12. The level ofthe liquid in the tank is an important factor in controlling processes.Besides requiring a measurement of the current level in the tank, it isalso necessary to know how fast the level is increasing or decreasing inorder to provide for satisfactory control across a range of processoperations.

The tank 10 has a conduit or fitting 13 near the bottom that is coupledto a bypass or tubular stand pipe 14. A suitable blocking valve 15 canbe provided in conduit 13, and of course suitable couplings can also bemade. Stand pipe 14 is connected at its upper end with a conduit 20 to aport 21. In the preferred embodiment, conduit 20 also includes ablocking valve 19. As illustrated, port 21 is above the normal level ofliquid 11 in tank 10 while port 16, at the bottom of stand pipe 14, isbelow the liquid level 12. When valves 15 and 19 are open allowing fluidcommunication between stand pipe 14 and tank 10, a level indicated at 22is established in the stand pipe 14 that will correspond to the level 12of the liquid 11 in tank 10.

This type of a stand pipe arrangement for determining levels of tank iswell known and, in addition, it is well known to use a float 26 insidethe tube or stand pipe 14 with a magnet 27 within the float. The magnet27 generates a magnetic field, and in conventional operations, is usedfor moving a magnetic material indicator shown schematically at 28inside a sealed glass tube 29 to provide a visual indication of level.This arrangement, however, does not provide signals to various processcontrollers, nor is it very accurate because of mechanical friction ofthe indicator 28 in tube 29.

Prior devices have been used for providing a continuous read-out ofliquid level. However, these devices are either inaccurate or not asresponsive as needed.

In the present invention as illustrated in FIG. 2, a magnetostrictivetransducer is indicated generally at 30. The magnetostrictive transducer30 shown is of the type described and claimed in U.S. Pat. No. 4,952,873which can be referred to for details of construction. Transducer 30comprises an outer protective tube 31 which houses an interior waveguide 33. The protective tube 31 is supported on a mounting head 34. Themounting head 34 includes a mounting flange 35 that supports, or issupported by the protective tube 31, and the head 34 has a connector 36thereon for connecting to external circuitry shown generally at 37. Thewave guide 33 has its outer or remote end anchored to the tube 31through a link 40, and a tension spring 42 that in turn is also attachedto an end plug or plate 43. The wave guide 33 is maintained under sometension. The base end of the wave guide at the head 34 is mounted in amounting block 45, such as by soldering the wave guide into the block.The mounting block 45 is supported onto an end plate or bracket 46 in asuitable manner which in turn is fixed to the flange 35.

Magnet 27 is oriented within float 26 such that the magnetic fieldgenerated by the magnet 27 passes through the tube 14, and theprotective tube 31, which are both constructed from nonmagneticmaterials, and further passes through wave guide 33. Electric currentpulses are sent out along the wave guide 33 from a pulse generator 37Aforming a portion of the electronic circuit 37, and a return line 38provides for a current return. The pulse generator 37A provides a streamof pulses, each of which is also provided to a signal processing circuit37B for timing purposes. When the current pulse travels down the waveguide 33 in direction from the mounting block 45, the current pulseprovides a field passing through the wave guide and when this fieldintercepts the magnetic field from magnet 27, it will induce a torsionalloading into the wave guide 33 under known principles and thus provide atorsional strain wave pulse in the wave guide. The strain wave is atwisting of the wave guide 33 which is transmitted through the waveguide back toward the mounting block 45. As the torsional pulse movesbetween the two portions of a mode converter shown at 47 and 48,respectively, the movement of provided sensing tapes indicated at 49will be sensed to generate an electric signal back to the processingcircuit 37B. By proper comparison of the time of start of a particularcurrent pulse and the time of return of the sonic torsional strain wavepulse back along wave guide 33 a distance of the magnet 27 from the modeconverter center line will be obtained as an output signal 37C throughthe circuitry 37.

This arrangement is fully described in U.S. Pat. No. 4,952,873.

In the present invention, the protective tube 31 is suitably supportedonto the tube 14 through the use of suitable clamps indicated at 50. Astandoff or support block 51 can be placed between the side wall of thetube 14 and the side wall of the protective tube 31, and then aband-type clamp 50 passed around the outside of the tube to clamp itsecurely to the tube 14. These straps 50 can be used in two or moreplaces as shown.

The output signal 37C will represent the level 12 of the liquid 11 in the process vessel 10. The level 22 inside the tube 14 and the level 12of the process vessel liquid will remain the same, as long as the valvesin conduits 20 and 13 remain open and the magnet 27 will provide themagnetic field to the wave guide 33 as described above to provide aprecise liquid level indication relative to a reference level at thehead of the transducer 34 so that a continuous, very accuratemeasurement of liquid level can be obtained. The magnetostrictiveprinciples are particularly adapted to use in the present installation,inasmuch as the wave guide 33 and its protective tube 31 can be mountedon the exterior of the tube 14, and do not have to be within thecorrosive process fluid or subjected to the temperatures and pressuresthat may be present in the process vessel 10.

The arrangement permits the sensor output to be used in two-wire 4-20 matransmitters for continuous, real time control.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A magnetic sight gage sensor for a vessel havinga bypass tube made of magnetic transmissive material connected to thevessel in such a manner that the level of a liquid in the bypass tubecorresponds to the level of liquid in the vessel, magnet means floatingin the bypass tube at a level dependent on the level of liquid in thevessel, wherein the improvement comprises a magnetostrictive transducermounted outside of the vessel on the exterior of said bypass tube and inproximity thereto, said magnetostrictive transducer comprising aprotective tube made of magnetic transmissive material, a wave guidewhich passes adjacent to and is spaced from said magnet means and ismounted in said protective tube, means for providing electrical pulsesalong said wave guide and for sensing return pulses caused byinterference of magnetic fields carried by the wave guide as a result ofthe electrical pulses and the magnetic field of the magnet means toprovide an indication of the level of the magnet means relative to areference.
 2. The sight gage sensor of claim 1 wherein the floatingmagnet means comprises a float member and a magnet supported in thefloat member.
 3. The sight gate sensor of claim 1 wherein saidprotective tube is directly supported on and secured to said bypasstube.